Wavelength Division Multiplexing (WDM) has been developed as one technique for achieving large-capacity optical communication networks. WDM can multiplex a plurality of optical signals with different wavelengths. In particular, in a WDM optical communication network, a WDM optical signal is transmitted that in which a plurality of optical signals with different wavelengths are multiplexed.
Each node in a WDM optical communication network includes, for example, a Reconfigurable Optical Add/Drop Multiplexer (ROADM) as a transmission device that processes WDM optical signals. The ROADM may drop an optical signal with a desired wavelength from a received WDM optical signal, and guide this optical signal to a client. The ROADM may add an optical signal received from a client to a WDM optical signal.
In addition, an optical communication network preferably includes a function to restore a path when a fault has occurred. A fault recovery function is achieved by, for example, a protection path.
FIG. 1 illustrates an example of fault recovery based on an OUPSR (Optical Unidirectional Path Switched Ring) function. In the example depicted in FIG. 1, a plurality of ROADMs 201 (201A-201D) form a ring network. A pair of optical fiber links connects ROADMs 201. Optical signals are transmitted bidirectionally between nodes through a pair of optical fiber links.
Assume that, in the optical communication network configured as described above, an optical signal is transmitted from a transponder 202A accommodated within the ROADM 201A to a transponder 202B accommodated within the ROADM 201B. In this case, using an optical splitter (SPL) 203, the ROADM 201A generates optical signals X1 and X2 by splitting an optical signal X transmitted from the transponder 202A. The optical signal X1 is transmitted from the ROADM 201A to the ROADM 201B through an optical transmission link 205. The optical signal X2 is transmitted from the ROADM 201A to the ROADM 201B through an optical transmission link 206. Thus, the ROADM 201B receives the optical signals X1 and X2. Then, the ROADM 201B selects one of the optical signals X1 and X2 using an optical switch 204, and guides the selected signal to the transponder 202B. Under this situation, assume that a work path has been established on the optical transmission link 205 and that a protection path has been established on the optical transmission link 206. In the example illustrated in FIG. 1, the ROADM 201B selects the optical signal X1, which has been received through the optical transmission link 205, and guides the selected optical signal X1 to the transponder 202B.
Assume that a fault occurs in the optical transmission link 205 while the communication described above is being performed. In this case, the optical signal X1 does not reach the ROADM 201B, and the ROADM 201B detects a “loss of light” or a “loss of frame” for the optical signal X1. By doing this, the ROADM 201B controls the optical switch 204 so as to guide the optical signal X2 received through the optical transmission link 206 to the transponder 202B. That is, switching from the work path to the protection path is performed to restore the path between the transponders 202A and 202B.
In this way, identical optical signals are transmitted through a work path and a protection path in the optical communication network depicted in FIG. 1. That is, while the optical communication network is being operated normally, a ROADM on a reception side receives identical optical signals through a work path and a protection path. Hence, when switching from the work path to the protection path has been performed due to detection of a fault, the path between the transponders is immediately restored.
A transmission device has been proposed that includes a work system interface unit, a protection system interface unit, and a transmission link switching unit that selects a signal supplied from the work system interface unit or the protection system interface unit (e.g., Japanese Laid-open Patent Publication No. 2001-119359). A transmission device has also been proposed that includes one protection line for a plurality of work lines (e.g., Japanese Laid-open Patent Publication No. 5-327674).
In recent years, a ROADM that includes a CDC (Colorless, Directionless, Contentionless) function (this may hereinafter be referred to as a “CDC-ROADM”) has been put into practical use as one technique for achieving a flexible network. The Colorless function can allocate a desired wavelength to respective client ports of a ROADM. The Directionless function can connect a client accommodated in the ROADM to a desired degree. The Contentionless function prevents collision between optical signals with the same wavelength.
The CDC function of a ROADM may be achieved using, for example, a multicast switch. The multicast switch may guide an optical signal input via a certain optical port to one or more desired optical ports. The multicast switch may also guide a plurality of optical signals with different wavelengths input via a plurality of optical ports to one desired optical port. In this case, the plurality of optical signals with different wavelengths are combined and output.
However, when an OUPSR is configured in an optical communication network in which a CDC-ROADM is implemented at each node, there may be some problems. The following describes problems of the prior art by referring to FIG. 2.
As illustrated in FIG. 2, each CDC-ROADM 211 includes a multicast switch 212. Assume that an optical signal is transmitted from a transponder 202A accommodated in a ROADM 211A to a transponder 202B accommodated in a ROADM 211B, as in the example depicted in FIG. 1.
In this case, the ROADM 211A generates optical signals X1 and X2 by splitting an optical signal X using an optical splitter 203. The optical signals X1 and X2 are guided to different optical ports of the multicast switch 212. In the example depicted in FIG. 2, the optical signals X1 and X2 are guided to optical ports C1 and C2, respectively. The ROADM 211A controls the multicast switch 212 in a manner such that the optical signal X1 is guided to an optical transmission link 205 and such that the optical signal X2 is guided to an optical transmission link 206. As result, the optical signal X1 is transmitted to the ROADM 211B through the optical transmission link 205, and the optical signal X2 is transmitted to the ROADM 211B through the optical transmission link 206. Meanwhile, the ROADM 211B controls the multicast switch 212 in a manner such that the optical signal X1 received through the optical transmission link 205 is guided to the optical port C1 and such that the optical signal X2 received through the optical transmission link 206 is guided to the optical port C2. The optical signals X1 and X2 respectively output from the optical ports C1 and C2 are guided to an optical switch 204. The ROADM 211B selects one of the optical signals X1 and X2 using the optical switch 204, and guides the selected optical signal to the transponder 202B. When a fault has occurred in the optical transmission link 205 for transmitting the optical signal X1, the ROADM 211B controls the optical switch 204 so as to perform switching from the work path to a protection path, as in the method depicted in FIG. 1.
In the configuration depicted in FIG. 2, however, two optical ports are used to transmit one optical signal. In particular, to transmit the optical signal X, the optical ports C1 and C2 of the multicast switch 212 are occupied in the ROADM 211A, and the optical ports C1 and C2 of the multicast switch 212 are occupied in the ROADM 211B. That is, the number of clients the ROADMs can accommodate decreases. Alternatively, the size of the multicast switch 212 needs to be increased. In addition, the switching function (multicast switch 212 and optical switch 204) in the receiving-side ROADM is arranged in two stages. In other words, the configuration of the ROADM is redundant.
Such problems may be solved in, for example, a configuration in which an optical signal is transmitted through only a work path during a normal operation (i.e., while a fault has not occurred) while an optical signal is transmitted through a protection path when a fault has occurred. However, such a configuration may extend a period of time needed to restore a path since an occurrence of a fault. For example a ROADM may include a wavelength selective switch (WSS) that processes an optical signal for each wavelength. Hence, an optical signal transmitted through a protection path corresponding to a work path in which a fault has occurred passes through one or more wavelength selective switches. A wavelength selective switch includes an optical attenuation function, and needs to adjust that attenuation function in a manner such that the power of a passing optical signal reaches a target level. However, adjusting the optical attenuation function of a wavelength selective switch takes a relatively long time (e.g., several hundred milliseconds). Therefore, in a configuration in which an optical signal starts to be transmitted through a protection path after a fault in a work path is detected, wavelength selective switches are adjusted after the fault has been detected, thereby leading to a long time to restore the path.